This invention relates, in general, to a communication device having a wideband receiver capability and an operating method therefor, and is particularly, but not exclusively, applicable to a communication system operating a universal frequency re-use pattern, such as deployed in a code division multiple access (CDMA) environment, in which the use of such a wideband receiver is required to recover broadband signals from a selection of available spectral bands.
Radio frequency (RF) communication systems offer an effective mechanism for supporting data and voice communications. Indeed, cellular RF systems can be quickly deployed to cover large geographic areas, with subscribers to the cellular service merely requiring a handset (or RF modem) to obtain access to the cellular network. This is in stark contrast with conventional wire-line communication systems that necessarily require individual line connections (in the form of twisted pairs or optical fibres) to be made to each subscriber terminal. In fact, the cost of deploying a RF-based cellular service is relatively inexpensive to terms of both time and cost when compared against a wireline system having a similar service capability.
The desirability of implementing RF communication systems is, however, tempered by the limited radio frequency spectrum that is available to support such services. Indeed, commercial cellular services, for example, do not have a uniform spectral frequency allocation on a global basis, with different countries assigning different spectral bands to the same form of service. Furthermore, commercial RF services, generally, are assigned frequency bands that are slotted in between military frequency systems, reserved frequency bands allocated for emergency services and other stellar, commercial or scientific frequency bands. Furthermore, in relation to the assignment of frequencies, national regulatory bodies (such as the Federal Communication Commission (FCC)) allocate radio frequency bandwidth for particular communication services. Indeed with respect to this allocation of frequency, the regulatory authority may not necessarily allocate a single block of spectrum to a particular service, but instead may assign discrete, smaller blocks of spectrum. Indeed, the smaller blocks of spectrum can be supplied from a combination of previously unused spectrum and now system-defunct spectrum that no longer supports a particular form of radio communication, e.g. low RF military application. Consequently, a supplier of infrastructure equipment, particularly, must provision for the subsequent release of radio spectrum for the stipulated communication protocol, e.g. a CDMA modulation scheme.
Consequently, cellular equipment manufacturers, generally, must necessary design systems and handsets that can be adapted (after initial deployment) to support new frequency bands subsequently made available to a network operator, while also having to manufacture equipment that operates at different frequencies. In this latter respect, a change in the operating frequency does not necessarily require a simple alteration in the receiver front end, but instead may require re-design of a significant portion of a transceiver to order to produce an operational unit at a different frequency. Clearly, any re-design of equipment is both costly and time consuming for the manufacturer.
Nevertheless, the popularity of RF-based systems is placing ever-increasing demands on the limited radio spectrum, and in this respect cellular communication systems have been developed that attempt to optimise that available bandwidth. For example, the global system for mobile (GSM) cellular communication systems operates a time division multiplexed scheme in which a carrier frequency supports a number of time multiplexed communication channels, with each carrier frequency framed into time slots.
Unfortunately, time division multiple access (and, for that matter, frequency division multiple (FDM) schemes, generally) necessarily operate frequency re-use patterns within the cellular system. More specifically, cells in the system have frequency carriers assigned to them (usually) on a permanent basis and in a way that interference between frequency carriers on an adjacent channel and co-channel basis is minimised. In other words, re-use of a first carrier frequency may be prohibited in adjacent cells so as to improve the radio environment by limiting potential interference (caused by a substantially identical frequency carriers corrupting the integrity of each others data).
In an attempt to further enhance capacity of time division multiplexed (TDM) systems, re-use patterns may, in fact, be on a sector basis, with each cell containing typically three or more sectors. In this way, lower power transmissions may be used, whereby interference from a particular frequency carrier is reduced (as a consequence of the effective transmission distances of these lower power signals) and carrier re-use hence increased. Furthermore, present TDM systems can operate frequency patterns that employ the underlay of microcells (or picocells) beneath macrocells. Again, such a system increases capacity, but still suffers from co-channel and adjacent channel interference.
A more efficient cellular communication scheme is the nineteen-hundred MegaHertz (MHz) personal communication system (PCS) operated in North America, which scheme operates a code division multiple access (CDMA) technique.
In contrast to TDM-based cellular systems, a CDMA system has a universal frequency re-use that allows frequencies to be used across the entire network, i.e. there is a frequency re-use of one. Such CDMA systems operate by virtue of the fact that a single carrier frequency supports a number of communication resources that are structured from discrete, coded sequences. More specifically, each channel is comprised from a unique coded sequence of xe2x80x9cchipsxe2x80x9d that are selected from a relatively long pseudo-random spreading sequence (typically many millions of bits in length). A communication device therefore has access to an information-bearing channel by virtue of a communication device having particular and detailed knowledge of a specific code that identifies the specific bits used by the information-bearing channel. More particularly, information (such as voice or data) is spread across many chips of the spreading sequence on a unique basis, with a processing gain of the system determined by the number of chips required to construct a single data bit. In this way, less than one bit of information is transmitted per chip.
CDMA systems therefore inherently operate in an interference environment because many channels utilise the same carrier frequency, with individual channels merely differing from one another in terms of their uniquely defined coded sequences. However, CDMA systems become statistically efficient for large populations of users, and therefore present an attractive and more efficient alternative to FDM-based systems.
CDMA systems must therefore necessarily impose and retain strict power controls on all transmissions, with this being particularly important in relation to transmissions from mobile communication devices. Unfortunately, CDMA systems are prone to operational instability in the face of xe2x80x9crogue mobilesxe2x80x9d in close proximity to base station transceivers and which rogues mobiles transmit at high power levels. As will now be appreciated, high-powered transmissions from the rogue mobile will swamp the universal frequency carrier and therefore corrupt information bearing chips, with this effect known as the xe2x80x9cnear-farxe2x80x9d problem. Indeed, the near-far problem can ripple-through and potentially unbalance the whole CDMA system to an extent where system-wide failure can result; this is clearly catastrophic for a network operator and must be avoided at all costs.
Other mechanisms that allow the radio spectrum to be utilised more efficiently include that concept of using lower bit-rate voice coders (termed xe2x80x9cvo-codersxe2x80x9d). Unfortunately, while increasing the number of available channels, low bit-rate vo-coders reduce the quality of the speech, and are hence less desirable to users because they impair the communication.
To date, infrastructure manufacturers (principally) have produced equipment that has separate receiver chains for each frequency in a CDMA system, i.e. different frequency carriers are applied to distinct receiver chains. This form of architecture is expensive to manufacture because each receiver chain must contain: a dedicated frequency oscillator for frequency down conversion; an intermediate frequency amplifier; an analog pass-band filter having a bandwidth appropriate for the frequency channel; a relatively low specification analog-to-distal (A/D) converter; and a back-end receiver channel having a digital filter and demodulation circuitry. As such, in order to allow a network operator some flexibility in system development, additional (and initially redundant) hardware may need to be incorporated into a base station at a time when it is unclear as to what (if any) additional blocks of frequency will subsequently be made available to the network operator. The architecture is therefore not only inflexible in terms of future system enhancement and development, but the provisioning of additional receiver chains increases the cost of the equipment, although its omission (while cheaper in the short term) may ultimately incur greater expense for the network operator as a consequence of the requirement for expensive in situ modification of a base station.
Alternatively, prior art wideband receivers utilise a local oscillator that down-converts incident radio frequency signals (in a mixer) to a range of selectable, intermediate frequencies. Again, the receiver chain will include an intermediate frequency amplifier and an analog filter, although the analogue filter will this time have a wide bandwidth commensurate with the bandwidth required to accommodate all possible frequency carriers. For example, the bandwidth of such a wideband filter would need to be at least 3.75 MHz in order to support three 1.25 MHz frequency carriers (ignoring the requirement for guard bands). However, in the alternative mechanism, a high specification (and therefore high-cost) A/D converter is required.
In relation to the alternative wideband mechanism of the prior art, only recent advances in filter technology (especially in relation to surface acoustic wave (SAW) devices) and A/D converter technology (particularly component manufacturing techniques) have made wideband architectures economically feasible and realisable. Notwithstanding the foregoing, multichannel receivers presently still have difficulty in efficiently meeting the requirements of TIA (Telecommunication Industry Association) interim standard IS-97 xe2x80x9cRecommended minimum requirements for base stations . . . xe2x80x9d because, in order to accommodate rogue mobiles, the system nevertheless requires sensitive gain-controlled and high specifications A/D converters.
U.S. Pat. No. 5,497,395 describes a system and method of communicating information within a digital communication system and especially a spread spectrum (code division multiple access) system. A receiver chain of the system contains a series arrangement of a receiver demodulator and an analog-to-digital converter, with this document providing background and framing the present invention in context.
EP-A-0 803 993 describes a transceiver arrangement that is arranged to switch traffic channels within spread spectrum beams of a satellite system. Specifically, a single frequency carrier supports a number of users, with processing restricted to individual channel recovery on that single carrier.
It would be desirable to produce relatively low-cost receiver chain for a communication device (such as a base station or a handset) that can accommodate multiple frequency carriers, while not employing the use of relatively circuitry or the physical duplication of the entire receiver chain for each carrier.
According to a first aspect of the present invention there is provided a receiver circuit for a communication device, the receiver circuit arranged to receive a plurality of carriers having relatively high but differing frequencies, the receiver circuit comprising: a frequency converter coupled to receive and arranged to down-convert the relatively high frequency carriers to a plurality of relatively low but differing frequency carriers; and a plurality of receiver chains response to the plurality of relatively low frequency carriers, the plurality of receiver chains each having a filter arranged to isolate selected ones of the plurality of relatively low frequency carriers.
In another aspect of the present invention there is provided a base station for a communication system having a universal frequency re-use scheme, the base station having a receiver circuit arranged to receive a plurality of modulated wideband carriers having relatively high but differing frequencies, the receiver circuit comprising: a frequency converter coupled to receive the plurality of modulated wideband carriers and arranged to down-convert the relatively high frequencies to a plurality of relatively low but differing frequency carriers; and a plurality of receiver chains response to the plurality of relatively low frequency carriers, the plurality of receiver chains each having a filter arranged to isolate selected ones of the plurality of relatively low frequency carriers.
In a preferred embodiment, the receiver circuit is arranged to receive at least three relatively high frequency carriers and wherein at least one of the receiver chains comprises a plurality of branches, each of the plurality of branches containing a filter arranged to isolate selected ones of the plurality of relatively low frequency carriers.
Preferably, the at least three relatively high frequency carriers occupy contiguous frequency bands, and wherein the plurality of branches contain filters that isolate carriers having a non-contiguous frequency band relationship therebetween and, more especially, a next but one frequency band relationship.
In another aspect of the present invention there is provided a method of isolating a plurality of carriers incident to a receiver of a radio frequency communication system having a universal frequency re-use pattern, the plurality of carriers having relatively high but differing frequencies, the method comprising the steps of: applying the plurality of carriers to a common frequency converter arranged to down-convert the relatively high frequencies to a plurality of relatively low but differing frequency carriers; and isolating selected ones of the plurality of relatively low frequency carriers by applying the plurality of relatively low frequency carriers to a plurality of receiver chains each having a dedicated band-pass filter.
The method preferably includes the step of isolating carriers by applying non-sequential carriers to a plurality of branches within the receiver chains.
Advantageously, therefore, the present invention provides a relatively low cost receiver chain that can be readily adapted to support a number of wideband carriers having stipulated frequency ranges. Moreover, the present invention has a flexible architecture that can accommodate subsequent carrier (channel) release. Furthermore, the partitioning of adjacent frequency channels between different branches of the receiver chain improves performance by enhancing isolation of potentially interfering signals.